1. Field of the Invention
The present invention relates to a wire saw device for cutting out a number of wafers from a silicon ingot, an ingot of a compound semiconductor material, or the like, and particularly to a wire saw device having a structure which allows wafer cut out from an ingot to maintain high quality.
2. Description of the Related Art
A wafer for use in a semiconductor substrate is produced by slicing a material such as a silicon ingot or an ingot of a compound semiconductor material. As a result of recent increase in size of such an ingot, the conventional cutting process of an ingot by an inner-diameter blade slicer has been increasingly replaced with a method of producing a large number of wafers by simultaneous slicing process using a wire saw device. A piece of a material, including such an ingot as described above, which is subjected to a cutting process by a wire saw, is generally referred to a “work”.
A wire saw device is a device, as disclosed in JP 09-262826 Laid-Open, for causing wires wound around rollers to run at a relatively high speed, injecting slurry from a nozzle to apply the slurry onto the wires and pressing the wires on a work in such a state as this, thereby cutting the work to simultaneously cut out a number of wafers.
A schematic structure of a standard wire saw device will be described hereinafter. A wire saw device mainly includes wires for cutting a work, plural rollers on which the wires are suspended such that the wires can be caused to run, a mechanism for imparting tension to the wires, a work holding portion for feeding the work downward, and a mechanism for supplying slurry to the wires during the wire cutting process. The wires are unwound from one wire reel and fed to the rollers via a traverser and a tension-imparting means such as a powder clutch, a dancer roller and the like. The wires constitute a group of multiple wires aligned in the roller axial direction and suspended over the plural rollers by being spirally wound over the rollers totally 300 to 400 times. Each roller is produced by injecting polyurethane resin to a peripheral portion around a steel cylinder and forming grooves on the resin surface at a constant pitch. The wound wires, driven by a driving motor, can make reciprocal movements in a predetermined period.
When a work is to be cut, the work holding means holding the work is moved toward the multiple wires and the work is pushed into the multiple wires running at a pre-programmed feeding speed. A nozzles is provided in the vicinity of the multiple wires so that slurry containing grinding particles and a dispersant can be supplied to the multiple wires from a slurry tank at the time of cutting. Further, a slurry chiller is connected to the slurry tank so that the temperature of slurry to be supplied can be adjusted. An ingot is sliced by using such a wire saw device as described above, specifically, by imparting appropriate tension to the wires by using the wire tension imparting means and causing the multiple wires, driven by the driving motor, to make reciprocal movements.
However, there is a problem in such a wire saw device as described above, in that slurry supplied to the wires changes the temperature of the ingot when the ingot is cut by the wires, whereby waviness components on a wafer surface increases and the quality of the obtained wafers deteriorates. Examples of the surface waviness components include nanotopography of projection-recess components in the range of 0.2 to several dozen mm, “Warp” as the maximum magnitude of warpage at a wafer surface, and the like. Among these examples, increase in nanotopography significantly affects the quality of wafers.
The nanotopography as described above represents a surface waviness component having shorter wavelengths than those of Warp and constituted of wavelength components where λ is in the range 0.2 to several dozen mm. Nanotopography as described above presumably affects the yield rate of the STI (shallow trench isolation) process in production of a device. Nanotopography is generated during processing steps (from slicing to grinding) of a wafer and nanotopography caused by slicing by a wire saw (i.e. slice waviness) is classified into three types of components: suddenly-generated component; component generated in the cut-starting or the cut-finishing portion of a wafer; and component having periodicity. Among these components, the component generated in the cut-starting or the cut-finishing portion of a wafer is likely to be judged as defective in the numerical judgment of nanotopography. In particular, nanotopography in the cut-finishing portion is generally larger than nanotopography in the cut-starting portion and thus likely to exhibit the worst numerical value of nanotopography in a wafer surface and be regarded as defective in the numerical judgment.
To address the aforementioned problem, JP 2008-078473 Laid-Open discloses a wire saw device in which the temperature of slurry to be supplied is controlled to thereby control change in temperature of an ingot to suppress increase in nanotopography caused by change in temperature of the ingot. In this case, since control of the temperature of slurry is effected only through the control of temperature of slurry supplied from a nozzle, the basic structure of the conventional wire saw device can be used as it is and thus no significant increase in cost is resulted therefrom. Alternatively, JP 2008-302618 Laid-Open discloses, as another solution to address the aforementioned problem, a wire saw device where slurry is divided into two systems, i.e. slurry for cutting and slurry for temperature adjustment, and the temperatures of the respective slurries are independently controlled so that change in temperature of an ingot during the cutting process is further meticulously controlled to more effectively suppress increase in nanotopography caused by change in temperature of the ingot. However, the wire saw device described in JP 2008-302618 needs two nozzles and the temperatures of slurries released from these nozzles have to be controlled, respectively, whereby the device structure is complicated and the device itself is quite expensive.
Further, when an ingot is cut by using such a wire saw device as disclosed in JP 2008-078473 or JP 2008-302618, slurry splashes and attaches to the top surface of an ingot during the cutting of the ingot. The slurry attaching to the top surface of the ingot exhibits increased viscosity due to evaporation of moisture caused by cutting heat and the like. Such slurry having increased viscosity is then supplied to portions of a work which have already been cut by the wires and flows into sites where the cutting operation is being carried out, whereby cutting precision deteriorates. Yet further, since the slurry splashed on the top surface of an ingot is supplied again to the ingot, slurry supply becomes too much, making it difficult to appropriately control the temperature of the ingot and thus increasing nanotopography and Warp, which results in deterioration of wafer quality.
As a means for solving the problems described above, JP 2007-273711 Laid-Open discloses a wire saw device having a catching member for catching slurry which splashes when being supplied from a nozzle. The catching member is fixedly assembled with a work holding portion. The device of JP 2007-273711 has an advantage in that it has the catching member and slurry splashed on the top surface of an ingot is caught, though not perfectly, by the catching member, whereby increase in nanotopography and Warp by slurry as described above is suppressed. Further, the wire saw device of JP 2007-273711 has a relatively simple structure, which does not so significantly increase the production cost.
On the other hand, JP 2009-113173 discloses a wire saw device including a plate- or block-shaped slurry splash preventing member which can move in the horizontal direction or make lateral movement along a circular orbit in accordance with the shape of a work. It has been reported that the wire saw device of JP 2009-113173 can suppress bellow-like movement of a work, which movement is caused when slurry attached to wires hits cut-in portions of the work, splashes and the splashed slurry reflows into the cut-in portions, and that the device therefore can improve the quality of a wafer in terms of Warp.
However, the wire saw device disclosed in JP 2007-273711 has a problem in that the catching member thereof cannot sufficiently catch slurry released from the nozzle and a portion of the slurry splashes on the top surface of the work, whereby nanotopography and Warp increase.
Further, in the case of the slurry splash preventing member of the wire saw device of JP 2009-113173, it is necessary to constantly monitor the relationship between the position of a work and the position of the slurry splash preventing member, to cause the slurry splash preventing member to move horizontally or make lateral movement along a circular orbit in accordance with the shape of the work. In order to cause the slurry splash preventing member to move upwardly with maintaining a state where the slurry splash preventing member is constantly in contact with the work, highly sophisticated control is required and it is difficult to employ such a device structure in actual practice. If the slurry splash preventing member is caused to make excessive lateral movement, the slurry splash preventing member may strike the outer peripheral face of a work and drop the work off the device. Further, as described above, a very expensive device is required to achieve such a precise device structure as enables causing the slurry splash preventing member to move upward with maintaining a state where the slurry splash preventing member is constantly in contact with the work.
In view of these facts, it is therefore necessary to provide a clearance between the work and the slurry splash preventing member in actual practice. However, in such a device structure as this, there is a possibility that slurry released from the nozzle is not completely blocked and a portion of the slurry splashes on the top surface of the work, deteriorating nanotopography and Warp. Specifically, when a portion of slurry splashes onto the top surface of a work, the temperature of the splashing slurry drops due to exposure of the slurry to the air and a portion of the slurry of which temperature has thus dropped splashes on the top surface of the work which has been heated due to the processing heat generated by the cutting operation, whereby a region of the work where the cooled slurry has splashed and the vicinities thereof suffer from rapid decrease in temperature and thermal shrink associated therewith. As a result, nanotopography and Warp deteriorate at the portions of the work which has experienced rapid thermal shrink.
In this regard, there has been a demand for a wire saw device capable of effectively preventing slurry from splashing onto the top surface of a work and also sufficiently suppressing increase in nanotopography and Warp. Further, there has been a demand for providing such a wire saw device as described above at a relatively cheap price.